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Abstract

Silicon nanoribbons (Si NRs) with a thickness of about 30 nm and a width up to a few
micrometers were synthesized. Systematic observations indicate that Si NRs evolve
via the following sequences: the growth of basal nanowires assisted with a Pt catalyst
by a vapor-liquid-solid (VLS) mechanism, followed by the formation of saw-like edges
on the basal nanowires and the planar filling of those edges by a vapor-solid (VS)
mechanism. Si NRs have twins along the longitudinal < 110 > growth of the basal nanowires
that also extend in < 112 > direction to edge of NRs. These twins appear to drive
the lateral growth by a reentrant twin mechanism. These twins also create a mirror-like
crystallographic configuration in the anisotropic surface energy state and appear
to further drive lateral saw-like edge growth in the < 112 > direction. These outcomes
indicate that the Si NRs are grown by a combination of the two mechanisms of a Pt-catalyst-assisted
VLS mechanism for longitudinal growth and a twin-assisted VS mechanism for lateral
growth.

Introduction

One-dimensional nanostructures have attracted much attention in the research community
owing to their novel physical and chemical properties and due to their easy manipulation
as building blocks for nanoscale devices. In particular, nanoribbons (NRs) are of
interest on account of their geometrical shape, comprised of a rectangular cross-section
on a nanometer scale that can provide unique properties for optical, mechanical, and
electrical devices. Limited experiments on III-V and oxide semiconductor NRs have
already shown promising properties, such as the wave-guiding of photons, lasing action,
nonlinear polarization, Aharonov-Bohm interference, and high mechanical flexibility
[1-5]. Meanwhile, it is highly advantageous for device application if the NRs can be fabricated
with a semiconductor compatible with the complementary metal-oxide semiconductor process.
A good example here is a semiconductor made of silicon (Si).

Two different methods to prepare Si NRs have been reported. The top-down approach
uses lithography and etching procedures to create the NRs from wafers, which affords
a well-defined morphology and crystalline orientation [6]. Meanwhile, the bottom-up approach uses chemical synthesis with molecular precursors
to synthesize the NRs by an oxide-assisted growth (OAG) or vapor-liquid-solid (VLS)
mechanism [7,8]. However, the fabrication of Si NRs via the bottom-up approach is still in its nascent
stage; developing reliable synthesis processes as well as understanding the growth
mechanism are crucial to exploit the potential of Si NRs. Herein, we report the synthesis
of Si NRs and their combinatorial growth mechanism consisting of a metal-catalyst-driven
VLS and a defect-driven vapor-solid (VS) mechanism.

Experimental procedure

Si NRs were synthesized on Si (111) substrates using CVD process. Conventional wet
chemical cleaning processes were performed to remove any residual components from
the substrates. Pt thin film (0.5 nm) was deposited as catalyst by using the electron-beam
evaporator. The substrates were then placed in a hot-wall horizontal reactor and heated
to the reaction temperature of 1,000°C under a H2 (99.9999%) and an Ar (99.9999%) flow of 100 and 100 standard cubic centimeter per
min (sccm), respectively. SiCl4 (Aldrich, 99.9999%, Aldrich Chemical Co., Milwaukee, WI, USA) was then supplied for
10 min by bubbling with H2 as a carrier gas at 20 sccm. The carrier gas was then turned off and the reactor was
cooled to room temperature.

The structural properties of the Si NRs were characterized using scanning electron
microscopy (SEM) (Hitachi 3000, Hitachi Co., Tokyo, Japan) and transmission electron
microscopy (TEM) (JEOL 7100, 200 keV, JEOL, Tokyo, Japan). To prepare the samples
for TEM observation, the NRs were dispersed via the ethanol solution. A small droplet
of the solution was then dropped onto the copper TEM grid. To prepare the samples
for cross-section TEM observation, the saw-like edged NRs were dispersed via ethanol
solution onto Ge substrates coated with 30 nm of Au film. The cross-sectional samples
were produced by FIB (Nova 600 Nanolab) and a lift-out technique (Figure S1 in Additional
file 1). The cross-sectional samples were affixed to TEM grid and sliced to electron transparency
with progressively smaller ion-beam currents.

Results and discussion

Si NRs were synthesized on Si substrates assisted by Pt as a catalyst via chemical
vapor transport system [9,10]. Figure 1a shows a SEM image, showing a large quantity of flexible Si NRs on the substrate.
Most of the NRs have a thickness between 30 and 40 nm, a width of a few micrometers,
and a length of a hundreds of micrometers (Figure 1a, b).

To address the growth mechanism, the evolution of Si NRs over time was examined by
TEM. While the degree of evolution differed from ribbon to ribbon, a general trend
could be acknowledged. Figure 2a-e shows the typical sequential evolution of the NRs with a processing time interval
of 2 min. Initially, Si basal nanowires grew, as shown in Figure 2a. Subsequently, the saw-like edges began to grow along the basal nanowires (Figure
2b-d), the interspaces between the saw-like edges filled, and eventually the NRs shown
in Figure 2e resulted. Our observation indicated that the triangular islands are distributed along
a ribbon uniformly, as shown in Figure 2b, c. Meanwhile, the average number of islands that is estimated from 15 ribbons is 9
± 3/μm. These indicate that the distribution of islands in a single ribbon is rather
uniform; however, is not quite uniform among different ribbons under same synthesis
conditions.

Figure 2.TEM images of NR. (a-e) TEM images showing the evolutionary stages of the NR; basal nanowire, saw-like edges
on the basal nanowire, and the NR.

To understand the crystal structure of the NRs, the saw-liked NRs were investigated
by TEM, as shown in Figure 3. The selected-area electron diffraction (SAED) pattern recorded along [-111] zone
axis (Figure 3b) indicated that the basal nanowires within the NRs grew along the < 110 > direction,
whereas the saw-like edges grew along the < 112 > direction. As shown in the inset
at the top of Figure 3a, no grain boundaries, misfit dislocations, or abrupt interfaces were observed at
the interface between the basal nanowire and the saw-like edges. This indicates that
the saw-like edges have an epitaxial relationship with the basal nanowires. The energy-dispersive
spectroscopy (EDS) analysis presented in Figure 3c shows that the NRs is free from impurities, including Pt.

Figure 3.HRTEM images of NRs. (a) HRTEM images showing the crystallographic orientation of the nanowire with saw-like
edges in the course of the conversion to the NRs. The inset at the top shows interface
between the basal nanowire and saw-like edge. The inset at the bottom shows the basal
nanowire. The scale bar in the images is 5 nm. Corresponding SAED pattern recorded
along the [-111] zone axis (b) and EDS spectrum (c).

To investigate the structure of NRs in detail, cross-sectional samples of the saw-like
edged NRs were prepared by focused ion beam (FIB) slicing and a lift-out process with
a micromanipulator (Figure S1 in Additional file 1). This was then observed by TEM. Figure 4a shows a TEM cross-section image of the as-grown Si NRs. The right side of the TEM
image in Figure 4a is the part of the basal nanowire, whereas the other side is the part of the saw-like
edge. The width of the saw is approximately 1 μm, and its thickness is about 35 nm,
as shown in Figure 1b and 4a-d. Further scrutiny of the morphology of the cross-sectional NRs shows no distinct
interfaces, which confirms the epitaxial relationship between the basal nanowire and
the saw-like edges. Figure 4b-d show cross-sectional high-resolution transmission electron microscopy (HRTEM) images
of the NRs, indicating that the basal nanowires have hexagonal cross-sections. Indeed,
< 110 > -oriented Si nanowires have been also shown to have hexagonal cross-sections
[11,12].

Figure 4.Cross-sectional TEM and HRTEM images of NR. (a) Cross-sectional TEM image of the saw-like edged NR. (b-d) Cross-sectional HRTEM images of the three regions (the end part of the saw-like
edge, the middle part of the saw-like edge, and the part of the basal nanowire) indicated
in panel (a). The insets of (b-d) show diffractograms of the Si region in the box
in each part. These indicate that the basal nanowire was grown along < 110 > direction
and that the Si nanosaw/NR is bi-crystalline containing a single {111} twin. (e) Schematic diagram of the projected shape and facets of the basal nanowire part.
(f) Schematic showing the formation of the Si NR.

It was interesting to note that the twin extending in the lateral growth direction
of the basal nanowires is oriented parallel to the < 112 > direction, as shown in
Figure 4b-d. The insets of Figure 4b-d show the fast Fourier transform (FFT) of the corresponding HRTEM images. The FFT
diffractogram in the inset of Figure 4d shows that Si NRs is bi-crystalline, containing a single {111} twin. The growth direction
of the basal nanowire is along < 110 > direction. According to the TEM outcome, the
structure of the basal nanowire can be depicted as shown in Figure 4e, where the < 110 > -oriented the basal nanowire exhibits a hexagonal cross-section
bounded by four {111} facets and two {100} facets with a single {111} twin. This twin
boundary extends along the < 112 > direction, which corresponds with the lateral growth
direction of the NRs.

Based on these results, the growth mechanism of Si NRs can be described as follows.
First, relatively thin Si nanowire with a diameter of 30 nm grows on the Si substrate
assisted by Pt as a VLS catalyst (Figure 2a). Our previous study of nanowires from the initial stage showed Pt catalyst at the
end of many nanowires [9]. The basal nanowires were grown in the < 110 > direction. This result stems from
the interplay of the liquid-solid interfacial energy with the Si surface energy expressed
in terms of the edge tension in this diameter regime of 30 nm [13-15]. The basal nanowires have twins that extend to the side edges. The formation of twins
in the nanowires has also been reported with Si or Ge nanowires grown in the < 112
> and < 110 > directions by the VLS or a supercritical fluid-liquid-solid (SFLS) mechanism
[16-19]. Twin formation in these cases occurs during nanowire nucleation and it extends down
the length of the nanowires as the nanowires grow because the twins can provide preferential
addition sites that maintain nanowire growth in the energetically favorable < 112
> or < 110 > direction.

It is noted that Pt catalysts have not been found in the NRs. This may occur due to
the etching out of Pt-Si liquid globules during the course of growth under a chloride
atmosphere. Because the chemical activity of the liquid metal globules becomes higher
as the diameter becomes smaller according to the Gibbs-Thompson effect, Pt-Si liquid
globules with a diameter of around 30 nm could be etched out after the initial stage
under chemically harsh conditions.

The twin appears to play an important role in the subsequent lateral growth (i.e.,
the growth of the saw-like edges) from the basal nanowires by the VS mechanism. In
fact, previous studies suggest that twins have critical roles in the crystal growth.
For example, the presence of a twin can drive the growth in a specific direction by
what is known as classical "reentrant twin mechanism" [20,21]. Indeed, the reentrant twin mechanism has already been suggested for the growth of
Si ribbons, which are very similar morphology to our Si NRs though the size were much
bigger (width of 30-150 μm and length of 1-20 mm) [20]. Here, {111} twin creates favorable nucleation sites at the growth interfaces and
atoms arriving from the vapor phase can readily accommodated at the nucleation sites,
which will drive a rapid net growth in the < 112 > direction.

Regarding the role of the twin on the lateral growth, it is also noted that the twin
creates distinct surface energy anisotropy in the basal nanowire. As shown in Figure
4e, the twinned Si nanowires have mirror-like crystal structures in which the two {100}
planes are adjacent on one side while the other four facets consisted of {111} planes.
The surface energy of the {100} facet is higher than that of the {111} facet [22]; thus, such a mirror-like crystallographic configuration results in anisotropic surface
energy states in a specific direction (i.e., the < 112 > direction). This type of
anisotropic surface energy can also induce preferred crystal growth at surfaces where
the surface energy is high (i.e., the direction of two {100} facets in the basal nanowires)
to minimize the surface energy associated with high-energy facets. Therefore, besides
reentrant mechanism, the twin could further drive lateral growth from the basal nanowires
by the VS mechanism by creating an asymmetric crystallographic configuration and thus
an asymmetric surface energy state. As mentioned earlier, Pt or other types of impurities
were not found in the saw-like edges or NRs. Hence, this lateral growth would occur
without the assistance of a metal catalyst.

The triangular configurations of the saw-like edges are due to the nucleation of two-dimensional
islands during the epitaxial growth on the Si (111) surface [23,24]. On the Si (111) surface, a triangular island can be formed by the slow growth rate
of two low-index step edge facets ([1-12] and [11,12]) inducing the formation of the triangular island. The subsequent process of the filling
of the saw-like edges may be due to anisotropic growth kinetics. As described, the
< 112 > directions are the fast growth directions. The sides of the triangles then
move quicker than the other orientation. The triangles form and < 112 > -oriented
facets grow out of the system leaving the {100} planar surface. In this case, the
width of the ribbon would be related to the density of the island nucleation sites
where large triangles will form when there are a few nucleation sites and the width
of the ribbon would be equal to the height of the largest triangle before it gets
in contact with another triangle. When the density of triangular islands is high,
the width of the ribbon would be smaller.

Figure 4f shows a schematic diagram that summarizes the evolution of Si NRs. As shown here,
the nanowires grow first along the < 110 > direction with a single {111} twin via
the VLS mechanism with a Pt catalyst. The saw-like edges then grow from the side of
the nanowire along the < 112 > direction via the twin-driven VS mechanism with further
filling of the edges by the selective condensation of vapor driven by the chemical
potential differences. Recently, free-standing Si nanosheets with a thickness of about
< 2 nm has been reported using similar synthesis conditions [25]. The difference between the nanosheets and nanoribbons reported here is growth mechanism,
wherein the former is grown by VS mechanism without catalyst while the latter is grown
by combinatorial VLS and VS mechanism using metal catalyst. By considering the potential
of catalyst and VLS mechanism for the control of morphology of Si nanostructures,
the combinatorial mechanism reported here may be helpful to create versatile one-dimensional
Si nanostructures.

Conclusion

The bulk of previous studies have reported the growth of one-dimensional Si nanostructures
(i.e., nanowires and NRs) via the VLS or the VS mechanism, respectively [8,15,26,27]. Our study implies that a combination of these two well-established growth mechanisms
makes it possible to prepare novel Si nanostructures such as Si NRs that can be used
for optical, mechanical, and electrical devices. Although the combinatorial approach
in this study only showed the growth of Si NRs, the concept of this approach can be
applied as a reliable process to prepare many other novel one-dimensional nanostructures.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

TEP carried out the main part of synthesis, the structural analysis, and drafted the
manuscript. KYL and IK participated in the structural analysis. JC participated in
the discussion of the cross-sectional TEM sampling. PV participated in the discussion
of the growth mechanism. HJC participated in the design of the study, draft preparation
and coordination. All authors read and approved the final version of the manuscript.

Acknowledgements

This research was supported by the Second Stage of Brain Korea 21 project in Division
of Humantronics Information Materials, a grant from the National Research Laboratory
program (R0A-2007-000-20075-0), Nano R&D program (2009-0082724), and Pioneer research
program for Converging technology (2009-008-1529) through the Korea Science and Engineering
Foundation funded by the Ministry of Education, Science and Technology.